A band-pass filter at the intermediate frequency amplifier stage of ordinary FM receivers does not have ideal frequency vs. amplitude/phase characteristics over a desired frequency band. Particularly, in the case where a modulating signal is a stereo composite signal, for example, the components of which the subcarrier of 38 kHz was carrier suppression amplitude modulated by a differential signal (L-R) for the right and left channels are easily subjected to distortion due to nonlinearity of the frequency vs. amplitude/phase characteristics of a band-pass filter at this intermediate frequency amplifier stage. These distortions depend upon the characteristic of the band-pass filter, and the second and third harmonic distortions are generally serious problems.
When right-channel and left-channel signals L and R are decoded by a stereo MPX circuit on the basis of the main signal (L+R) and the differential signal (L-R) modulated subsignal which was subjected to distortion, distortion occurs in output signals L and R in the right and left channels. Therefore, although a method has been tried to improve linearity over a wide range of the frequency vs. amplitude/phase characteristics of the band-pass filter at the intermediate frequency amplifier stage, its effect is unsatisfactory or there is a problem such that an apparatus to realize such a purpose can be very expensive.
It is, therefore, a general object of the present invention to provide a stereo MPX circuit which can reduce the distortion to be caused in the right-channel and left-channel signals obtained from the stereo composite signal which was subjected to distortion.
A more specific object of the present invention is to provide a stereo MPX circuit which can set off the distortion to be caused in the right-channel and left-channel signals obtained from the stereo differential signal which was subjected to second or third harmonic distortion.